The band gap characteristics of certain materials such as gallium arsenide and indium phosphides and their ternary and quaternary alloys such as indium-gallium-arsenide-phosphide make them particularly suitable in the fabrication of semiconductors for photonic and optoelectronic applications such as e.g. lasers and diodes. One method of fabrication is to grow one or more layers epitaxially on a single crystal substrate such that the crystal structure of the deposited material effectively constitutes an extension of the crystal structure of the substrate.
Molecular beam epitaxy (MBE) is a known method of epitaxially depositing a single monolayer of high quality and purity material having a crystalline lattice structure which matches that of the substrate. The monolayer is deposited by directing a collimated beam of molecules from a source material onto the substrate in a vacuum environment. The thickness of the layers grown on the substrate can be as thin as one half of an atomic layer in thickness. The beam of molecules are generated in a furnace commonly referred to as a molecular beam epitaxy effusion cell which functions as a sublimator for sublimating source material to the gaseous phase. At present an internally heated crucible with a large exit aperture is used as the furnace of the molecular beam epitaxy effusion cell. The molecular beam is not fully controllable and accordingly instability exists and the coating deposition may lack the desired stoichiometry. These problems are manifested as a result of flux transients due to the inability to control overshooting and undershooting of the molecular beam from the effusion cell. This lack of control over the effusion flux and its dependency upon thermodynamic thermal adjustment affects the hyper abrupt stoichiometry of deposition and has limited the utility of the molecular beam epitaxy (MBE) technique primarily to laboratory applications in favor of other methods of deposition.